This invention relates to an illumination system and, more particularly, to an illumination system suitably usable in an illumination optical system of an exposure apparatus for manufacture of semiconductor devices, which uses as a light source an excimer laser of a vacuum ultraviolet wavelength region.
In a lithographic process among semiconductor device manufacturing processes, an exposure process for transferring, by exposure, a very fine pattern such as an electronic circuit pattern formed on the surface of a mask onto a wafer is prepared plural times, whereby electronic circuits are produced on the wafer.
As regards the exposure method used in the exposure process, there is a method in which a mask surface is held in contact with or in close proximity to a wafer surface and, in this stage, the mask surface is illuminated so that the pattern of the mask is transferred to the wafer surface. Also, there is a method in which a mask (reticle) placed at a position optically conjugate with a wafer surface is illuminated, and a pattern formed on the mask surface is transferred onto a wafer surface, by projection exposure, through a projection optical system. In any exposure method, the image quality of a pattern transferred to a wafer surface is largely influenced by the performance of the illumination system, for example, the uniformness of an illuminance distribution on the surface to be illuminated.
FIG. 10 is a schematic view of a general structure of a conventional illumination system, and it includes an inner type integrator and an amplitude division type integrator such as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 270312/1998.
In FIG. 10, laser light emitted by a laser light source 101 is once converged by a collimator lens 102 and then diverged, or alternatively, it is directly diverged by a negative lens, and the light is incident on an inside reflection surface of an optical pipe 103 at a predetermined divergent angle.
The divergent laser light having a divergent angle passes through the inside of the optical pipe 103 while being reflected thereby, and a plurality of apparent light source images of the laser light source 101 are produced on a plane (e.g., plane 110) perpendicular to the optical axis. Here, the laser beams, which appear as if they are emitted from the apparent light source images, are superposed one upon another on the light exit surface 103xe2x80x2 of the optical pipe 103, such that a surface light source of uniform illuminance is produced at the light exit surface 103xe2x80x2. The light beam from the light exit surface 103xe2x80x2 is directed by way of a condenser lens 104, an aperture stop 105 and a field lens 106, to the reticle 107 surface. Since the light exit surface 103xe2x80x2 of the optical pipe 103 is in an optically conjugate relation with the reticle 107 surface, the reticle 107 surface is also illuminated uniformly.
If the shape of the optical pipe 103 is determined while taking into account the length and width of the optical pipe 103 and the divergent angle of the laser light provided by the collimator lens 102, the optical path differences of each laser beam emitted from the apparent light sources at the plane 110 toward each of the points on the reticle 107 surface can be more than the coherence length of the laser light. This reduces the coherence with respect to time, and prevents speckle on the reticle 107 surface.
However, in the structure of the illumination system shown in FIG. 10, a change in position of the light, to be caused frequently as a result of using a laser light source, may produce a variation in incidence angle of the light upon the surface being illuminated. This results in non-uniformness of illuminance, upon the surface being illuminated.
Further, in the illumination system of FIG. 10, multiple reflection is made at the inside reflection surface of the optical pipe so as to increase the number of apparent light source images, by which uniform illuminance is attained. However, in order to assure the illuminance uniforming effect, the reflection times should be increased and, therefore, the optical pipe should have a sufficient length. Since, however, in the vacuum ultraviolet region, absorption of light by the glass material, that is, a decrease of transmission factor, is large, if an optical pipe of a length that assures sufficient illuminance uniformness is used, the efficiency of light utilization may be degraded as a result of it.
It is accordingly an object of the present invention to provide an illumination system by which uniformness of a light intensity distribution upon a surface to be illuminated can be maintained even if light from a light source changes, and also by which the efficiency of light utilization is improved.
In accordance with an aspect of the present invention, there is provided an illumination system for illuminating a surface by use of light from a light source, said illumination system comprising: an emission angle conserving optical unit effective to emit the light from the light source at a constant divergent angle; and a diffractive optical element for producing a desired light intensity distribution on a predetermined plane; wherein said diffractive optical element is disposed at or adjacent to a position where light from said emission angle conserving optical unit is collected.
In one preferred form of this aspect of the present invention, the illumination system may further comprise a multiple-beam producing element, and a light projecting element for superposing light beams from said multiple-beam producing element one upon another on the surface to be illuminated, wherein the predetermined plane corresponds to a light entrance surface of said multiple-beam producing element.
The illumination system may further comprise a zoom optical system for projecting the light intensity distribution, produced by said diffractive optical element, upon the light entrance surface of said multiple-beam producing element at a predetermined magnification.
There may be a plurality of emission angle conserving optical units of different divergent angles, and wherein said emission angle conserving optical units are interchangeably set at a light path in accordance with a change in magnification of said zoom optical system.
An emission angle conserving optical unit placed at the light path may be changed by another, whereby a numerical aperture of light incident on the light entrance surface of said multiple-beam producing element is substantially registered with a preset numerical aperture of said multiple-beam producing means.
There may be a plurality of diffractive optical elements for producing different light intensity distributions on the predetermined plane, wherein said diffractive optical elements are interchangeably set at a light path to produce a desired light intensity distribution on the predetermined plane.
The diffractive optical element may be a phase type or amplitude type computer hologram.
The emission angle conserving optical unit may comprise a fly""s eye lens having small lenses arrayed two-dimensionally.
The emission angle conserving optical unit may comprise an aperture and a lens system.
In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: an illumination optical system for illuminating a mask surface, as a surface to be illuminated, with use of light from a light source, said illumination optical system including (i) an emission angle conserving optical unit effective to emit the light from the light source at a constant divergent angle, and (ii) a diffractive optical element for producing a desired light intensity distribution on a predetermined plane, wherein said diffractive optical element is disposed at or adjacent to a position where light from said emission angle conserving optical unit is collected; and a projection optical system for projecting a pattern formed on the mask surface, as illuminated with the light from said illumination optical system, onto a wafer.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: applying a photosensitive material to a wafer; illuminating a mask surface, as a surface to be illuminated, with use of light from an illumination optical system, wherein the illumination optical system includes (i) an emission angle conserving optical unit effective to emit the light from the light source at a constant divergent angle, and (ii) a diffractive optical element for producing a desired light intensity distribution on a predetermined plane, wherein the diffractive optical element is disposed at or adjacent to a position where light from the emission angle conserving optical unit is collected; projecting, through a projection optical system, a pattern formed on the mask surface onto a wafer; and developing the transferred pattern.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.